In a large pot, heat the olive oil over medium heat. Add the onion, garlic, carrots, and celery and cook until the vegetables are tender, about 5 minutes.

Add the chicken to the pot and cook until browned on all sides, about 5 minutes.

Pour in the chicken broth and add the thyme and oregano. Bring to a boil, then reduce heat and simmer for 20 minutes, or until the chicken is cooked through and tender.

Remove the pot from the heat and allow it to cool slightly. Using an immersion blender or regular blender, puree the soup until smooth.

Return the pot to the heat and stir in the yogurt. Cook over low heat until heated through, about 5 minutes. Season with salt and pepper to taste.

Serve the soup hot, garnished with fresh parsley.